High security documents such as banknotes have substrates formed from various materials. In the United States, paper currency is made from a non-woven combination of 75% cotton and 25% linen fibers. In most other countries, pulp-based substrates are used. Some countries, such as Canada, have used cotton and paper blended banknotes. In addition, countries such as Australia, New Zealand and Canada have issued banknotes having polymer substrates, e.g., substrates including biaxially oriented polypropylene. The substrate, which may include one or more plies of the substrate material, may include security features such as laminated polymer or paper security threads, planchettes, and watermarks formed directly into the substrate. For example, U.S. paper currency contains small segments of red and blue fibers scattered throughout for visual identification.
Banknotes also include visual data printed on the substrates. The visual data may include images such as portraits, authentication information such as serial numbers, or both. The inks used to print on the substrates may include special dry color pigments blended with oils and extenders and phosphor chips containing layered micro-interference layers. Such inks include Flexo inks, gravure inks, and thicker intaglio inks.
High security documents such as banknotes are generally formed on substrate materials that are frequently equipped with security elements, which are difficult to imitate and which permit even a layman to check the authenticity of the printed information or the document. Security elements can be, for example, windowed security threads, which are visible in certain areas on the surface of the banknote, applied foils, which have a transparent or metallized embossed hologram, blind embossings, so-called “latent images” produced by printing technology or by printing and embossing technology, which render different information from different viewing angles, prints containing optically variable pigments and producing different color effects depending on the viewing angles, and prints comprising metallic effect ink, which have metallic luster, for example, in a gold, silver or bronze tone. In addition to these unaided features, there are quasi-public security threads, fibers and inks, which fluoresce or phosphoresce under illumination with ultraviolet (“UV”) or infrared (“IR”) sources.
Other security features in paper currency include numeric watermarks, Guilloche patterns, which are narrow geometric patterns created by a geometric lathe or mathematically, microprinting, digital watermarks, magnetic inks and threads, demetalized security threads, holographic features, fluorescent inks, lenticular lens array security threads, and fluorescent and non-fluorescent security threads.
High level covert security features include ENIGMA (De La Rue International) and M (Geiseke and Devrient). An important security feature in currency is the M feature, where “M” refers to “machine readable.” The M feature is a colorless, inorganic oxide integrated into the paper substrate, the printing ink, security ink, or a security thread, without causing any change in the appearance of the banknote. The powdered M feature may be blown into the paper substrate in a trail to identify a particular banknote denomination. When exposed to a flash from a strong source of light, the M feature emits a band of light in a split second that rapidly disappears. This repeatable, characteristic light band of the banknote can be authenticated by a reading device. The central banks protect the security of the M feature by requiring the use of special sensors to recognize it.
As counterfeiters have become more sophisticated, the security features in such documents have had to become more advanced as well in order to prevent widespread fraud. As the substrates of such secure documents have become more advanced, the cost to produce them has also increased, thus making the replacement of worn currency quite expensive. Therefore, it is important that in addition to being secure, such documents must have a high level of durability.
Banknotes are removed from circulation for a variety of reasons. Based on one study, 81% of notes are removed because of soiling, 9% are removed because of damage caused by mechanical means, especially tearing, 5% are removed because of graffiti on the notes, 4% are removed because of general wear and tear, and 1% are removed because of damage to the security elements. Generally, 60% to 80% of all rejected bank notes result from to soiling.
Banknotes have a finite time in circulation due to soling and tearing of the notes in use by the public. For example, it takes about 4,000 double folds (first forward and then backward) before a U.S. paper bill will tear. Banknotes are handled in many ways during their usable life and experience a variety of mechanical stresses, as well as being brought into contact with substances that can dirty the notes, resulting in difficulty in their authentication and use. One of the major determinants of the banknote life, which is shortest for the lowest denominations, is soiling. Work by the Dutch National Bank has shown that the primary source of soiling is deposited sebum following contact with fingers, which sebum eventually oxidizes and becomes yellow. Further, a study by the Microbiology Department of Karachi University in Pakistan concluded that currency notes could also carry contaminants that cause diarrhea and urinary tract infections, in addition to skin burning and septicaemic infection. One study found that 26% of notes contained high levels of bacteria, and 80% of notes had some traces of bacteria. An even more concerning finding was that pathogens, including bacteria and viruses, on banknotes have the potential to develop resistance to antibiotics, making the treatment of infectious diseases more difficult.
Such “dirty” money is not simply confined to developing nations. Some of the studies on contaminated currency emerging from the United States were equally revealing. In a recent survey conducted for the Department of Endocrinology at the Wright-Patterson Medical Center in Ohio, researchers collected 68 one-dollar notes from a concession stand at a high school sporting event and a grocery store check-out counter, and examined them for bacterial contamination. Only four bills (six percent) contained no detectable germs.
Given the huge amounts of banknotes in circulation for even small countries, determining the fitness of banknotes is not only of importance in cost control, but also poses a serious technical challenge in terms of processing speed and accuracy. Moreover, the extent of dirtiness of a banknote cannot easily be captured in objective rules. As a result, not only is accurate determination of the fitness of banknotes of interest from a cost point of view, but also cleaner notes are more secure and more attractive to the public. Studies have shown that soiling is one of the primary reasons for classifying banknotes unfit for circulation by banknote fitness sensors using both white light and specific wavelength sources.
In order to improve durability and soil resistance of these substrates, it is known to use documents of value with a dirt-repellent and/or moisture resistant protective layer to extend the documents' lifetime and fitness for circulation. Such a protective layer typically contains cellulose ester or cellulose ether for the greater part and micronized wax for a lesser part, and is applied all over the banknotes. The micronized wax is dispersed by kneading or mixing with oil, an ink binder or a mixture thereof. The sheets freshly printed with the protective layer can be stacked without difficulties and without any black ink from one sheet staining the sheet below.
Another coating composition containing only a binder and no fillers has been applied to the banknote paper, which has a large surface area or high surface roughness due to its porosity. The composition is applied in a layer and has a thickness with a smooth surface, thus having little possibility for resulting dirt deposits. Further, the coating is thin enough not to impair the other stated properties of the paper.
A problem with this approach is that known protective layers do not last or wear well. Conventional protective layers comprising water-based lacquers usually fail to completely meet a demanding requirement profile. For example, very good dirt repellence and adhesion qualities contravene resistance to the penetration of liquid, and vice versa. Water-based lacquers, therefore, currently meet the high requirements for a protective layer in security printing, and in particular banknote printing, only if a second component in the form of a crosslinking agent is added.
Another problem relating to banknotes is that central banks need to replace worn and soiled notes at a cost to taxpayers. In the United States, the volume of notes manufactured is in the billions of notes per year (4-6 billion typically). The production of banknotes is costly, particularly so for the higher denominations, which have many security features that are both accessible to the public and machine readable by bill acceptors and the central banks using high speed sorters. Banknote sorters made by Geiseke and Devrient, De La Rue International and Toshiba typically process banknotes at rates of 10-40 banknotes/second and perform a number of diagnostics using sensors in the notes' travel path. These sensors are a combination of authentication sensors as well as note fitness sensors. The fitness sensors primarily use imaging and analysis of the captured images to determine if the banknote should be destroyed or returned to circulation.
The cost of replacing banknotes is significant as the higher denominations contain Level I, II and III security features for use by the public, commercial banks, single note acceptor devices and central banks. In the United States, for example, the currency replacement budget is $747 million and breaks down as follows:                $1 and $2 notes—5.2 cents per note        $5 and $10 notes—8.5 cents per note        $20 and $50 notes—9.2 cents per note        $100 note—7.7 cents per note        $100 note to be released in October 2013—13 cents per note        
With over 150 billion new banknotes being manufactured and printed every year around the world, the cost of replacement of unfit currency has approached $10 billion annually. In addition to the replacing the notes, there is a sizable waste disposal cost associated with the destruction of the shredded notes that are determined to be unfit. This amounts to about 150,000 tons of waste worldwide annually, based on total worldwide circulation of 150 billion notes. This is particularly problematic for polymer notes, which also pose larger environmental problems with respect to burning and landfill disposal.
Based on these facts, there is a need to employ a manner for cleaning banknotes, which are soiled but not torn or ripped, that does not attack the print and security features of the note. There is still a further need for a system that applies a certain class of fitness parameters to cause identified banknotes to be cleaned using a method that does not attack the print and security features before making a determination that they should either be returned to circulation or destroyed.